The process of emulating the functionality of a first computer platform (the “target system”) on a second computer platform (the “host system”) so that the host system can execute programs designed for the target system is known as “emulation.” Emulation has commonly been achieved by creating software that converts program instructions designed for the target platform (target code instructions) into the native-language of a host platform (host instructions), thus achieving compatibility. More recently, emulation has also been realized through the creation of “virtual machines,” in which the target platform's physical architecture—the design of the hardware itself—is replicated via a virtual model in software.
Two main types of emulation strategies currently are available in the emulation field. The first strategy is known as “interpretation”, in which each target code instruction is decoded in turn as it is addressed, causing a small sequence of host instructions then to be executed that are semantically equivalent to the target code instruction. The main component of such an emulator is typically a software interpreter that converts each instruction of any program in the target machine language into a set of instructions in the host machine language, where the host machine language is the code language of the host computer on which the emulator is being used. In some instances, interpreters have been implemented in computer hardware or firmware, thereby enabling relatively fast execution of the emulated programs.
The other main emulation strategy is known as “translation”, in which the target instructions are analyzed and decoded. This is also referred to as “recompilation” or “cross-compilation”. It is well known that the execution speed of computer programs is often dramatically reduced by interpreters. It is not uncommon for a computer program to run ten to twenty times slower when it is executed via emulation than when the equivalent program is recompiled into target machine code and the target code version is executed. Due to the well known slowness of software emulation, a number of products have successfully improved on the speed of executing source applications by translating portions of the target program at run time into host machine code, and then executing the recompiled program portions. While the translation process may take, e.g., 50 to 100 machine or clock cycles per instruction of the target code, the greater speed of the resulting host machine code is, on average, enough to improve the overall speed of execution of most source applications.
Whether the target code is interpreted or translated, it is likely that the host machine will execute the resulting interpreted or translated instructions at a different rate than the target machine would execute the original target instructions. Consequently, the host machine may run faster or slower than the target machine being emulated. Such differences in execution speed may be tolerable—or even desirable—in programs like word processors and spreadsheets. However, these differences in execution speed are a significant issue for timing-critical operations like: (1) sound and video playback; (2) processing “streaming” information, where data is delivered to the processor at a constant rate; and (3) games and animations which require screen updates to display motion accurately.
Different rates of execution for the target and host machines may be addressed in software, e.g., by adjusting the execution rate of the host machine. For example, U.S. Pat. No. 6,882,962 to Linden describes a method for simulating the timing characteristics of a target platform designed for consistent instruction execution speed by measuring, predicting and dynamically adjusting for timing variability within a host platform. This technique uses an arbitrary “time quantum” as a referent that is multiplied by the target system's instruction cycle execution speed to determine the number of instructions the target system executes in a specified time. When non-native code is executed on the host system, a counter is used to track the number of instructions executed and to interrupt when a target number is reached. A processor-activity-independent timing source is queried to determine the time elapsed. The elapsed time is then compared to the original “time quantum.” The resulting ratio is a timing reference that is independent of the operating speed characteristics of any particular host system. This reference is used to predict the operational speed of the host system and to adjust factors in the host computer and emulation process to more accurately match the target system before executing the next block of instructions and repeating the process.
Although this system may work where the target instruction execution speed is consistent it does not address situations where different parts of the target system are emulated in different ways and have different clock rates. For example, one component of a target system may be emulated by interpretation, in which case the interpreted target instructions are run on the host system with a fixed clock. A different component of the target machine, however, may be emulated by translation, in which case the translated target instructions are run on the host system using a variable clock. Management of these two different clocks on the host system presents entirely different problems.
Thus, there is a need in the art, for emulation systems and methods that address these problems.